Power integrated circuits (PICs) carry current, typically DC current, to electric motors, lamps, and other devices that use electric power. More sophisticated PICs use multiple output PIC devices on a single PIC chip. These PIC devices produce heat as they provide power to various PIC loads.
In some instances, such as in a short-circuit condition, PIC devices may reach excessive temperatures that, if not limited, can damage the entire PIC chip. To address this problem, many PICs devices include thermal shutdown circuitry to cause the PIC device to shutdown when it reaches a thermal shutdown temperature setpoint. After shutting down, the PIC device may execute a hysteresis program to oscillate in temperature between the thermal shutdown temperature setpoint and a lower temperature setpoint. Thus, when thermal shutdown of the PIC device occurs, the current through that device switches to a shutdown current level, which may be zero current, to permit the device to cool to the lower temperature setpoint. Once the device reaches the lower temperature setpoint, circuitry may restore PIC device output current until the device again reaches the thermal shutdown temperature setpoint. This PIC device current-temperature hysteresis will continue until conditions exist that may cause it to stop.
Because a PIC chip often contains multiple outputs that radiate heat to the chip, it may be difficult to thermally separate the PIC devices from one another. For example, the heat that one output generates may affect the operation of other outputs. If one of the PIC outputs operate within the above-described current-temperature hysteresis, the average temperature of the oscillating PIC output may increase the temperature of adjacent outputs on the PIC chip to make them also execute their own current-temperature hysteresis.
Seeking to overcome this problem, some PIC chips separate the thermal shutdown circuits of the PIC devices as physically far as possible from the other thermal shutdown circuits on the chip on the PIC chip. This, to some degree, thermally separates the temperature sensing of the PIC outputs. However, separating the thermal sense circuits does not eliminate the thermal effects of an overheating PIC output from other properly operating PIC outputs on the common PIC chip.
The thermal separation problem may be seen in the following example. Suppose that a PIC output operates within a current-temperature hysteresis. If the thermal shutdown temperature setpoint is 150.degree. C. and the lower temperature setpoint for restoring current through the PIC device is 130.degree. C., an average PIC device temperature while performing the hysteresis may be 140.degree. C. Poor heating-sinking capability in the PIC chip may cause adjacent PIC outputs to experience temperatures as high as 100.degree. C., for example, from the failing PIC output. If the normal operating temperature of the adjacent output devices on the PIC chip approximates 70.degree. above ambient, for example, then the addition of the radiated 100.degree. C. from the over-heated PIC chip may cause the otherwise normally operating PIC device to experience temperature of approximately 170.degree.. This will cause the properly operating PIC device to activate its thermal shutdown circuitry.
An even more serious problem is observable in the following example. A short-circuited condition may exist in a PIC device that does not cause the PIC device to reach the 150.degree. C. thermal shutdown temperature setpoint. For example, the temperature in the short-circuited PIC device may reach 148.degree. C., but no higher. This temperature will cause the short-circuited PIC device not to execute the current-temperature hysteresis. From this high temperature an adjacent PIC device may still receive as much as 100.degree. C. of temperature through poor heat-sinking of the PIC chip. When added with the normal 70.degree. C. operating temperature of a properly operating PIC device, the total temperature may cause the normally operating PIC device to itself reach the thermal shutdown situation. This, again, will cause the normally-operating PIC device to undergo the hysteresis and indicate a failure mode. Thus, with a high average temperature from the failing PIC output, a non-failing PIC output may improperly indicate a failure condition.
As a result of the above problems, there is a need for a method and apparatus that improves the thermal independence between outputs of a multi-output power integrated circuit.
There is a need for a method and system that thermally separates a power integrated circuit device from a plurality of other such devices on a common PIC chip.